One function of a data storage device such as a disc drive is reliable storage and retrieval of information. Using one common implementation of a disc drive as an example, data is stored on one or more discs coated with a magnetizable medium. Data is written to the discs by one or more transducers, typically referred to as read/write transducers, mounted to an actuator assembly for movement of the transducers relative to the discs. The information may be stored on a plurality of concentric circular tracks on the discs until such time that the data is read from the discs by the read/write transducers. Each of the concentric tracks is typically divided into a plurality of separately addressable data sectors. The transducers are used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the transducer senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the transducer over the center of the desired track.
Conventionally, the transducers are positioned with respect to the disc surfaces by an actuator arm controlled through a voice coil motor. The voice coil motor is responsible for pivoting the actuator arm about a pivot shaft, thus moving the transducers across the disc surfaces. The actuator arm thus allows the transducers to move back and forth in an arcuate fashion between an inner radius and an outer radius of the discs. The actuator arm is driven by a control signal fed to the voice coil motor at the rear end of the actuator arm. A servo control system is used to sense the position of the actuator arm and control the movement of the transducer above the disc using servo signals read from the servo segments on the disc surface in the disc drive. The servo control system relies on servo information stored on the disc. The signals from this information generally indicate the present position of the transducer with respect to the disc, i.e., the current track position. The servo control system uses the sensed information to maintain transducer position or determine how to optimally move the transducer to a new position centered above a desired track. The servo system then delivers a control signal to the voice coil motor to rotate the actuator arm to position the transducer over a desired new track or maintain the position over the desired current track.
As the demand for smaller disc drives increases, so does the demand for higher storage capacities. To meet this demand, manufacturers of disc drives are continually developing smaller yet higher storage capacity drives. Typically, to increase the storage capacity of a disc drive, the density of the concentric tracks on the disc is increased. In order to increase the track density, manufacturers either narrow the width of the concentric tracks or reduce the spacing between tracks. However, these means of increasing track density are limited by the precision of the actuator and voice coil motor assembly.
Manufacturers have developed dual-stage actuators to increase the positioning accuracy of the read/write head. A dual-stage actuator includes the primary stage actuator controlled with a voice coil motor (as discussed above) and a microactuator controlled with a driving circuit. The microactuator may include one or more piezoelectric elements attached, coupled, bonded or integrated with the primary actuator. A piezoelectric element usually contains multiple layers of crystals. Applying a voltage potential across a portion of the crystal changes the dimensions of each crystal, and therefore, the piezoelectric element. Modern piezoelectric elements, or devices, are usually constructed of ceramic composites that exhibit piezoelectric characteristics. The ceramic composites are easily formed as thin layers on silicon substrates and integrated into electrical devices, such as microactuators.
Typical piezoelectric microactuators use “bimorph” piezoelectric elements made of two or more opposed piezoelectric strips that operate in opposition, i.e. one is extended while the other is contracted. This allows the elements to bend in response to an applied voltage. Indeed, in typical designs most piezoelectric elements are structural elements of the microactuator and the bending action produced is desired or even necessary for the function of the microactuator. However, bimorph piezoelectric elements are inherently more expensive than single piezoelectric elements to produce.
Accordingly there is a need for a microactuator design that utilizes a single piezoelectric element. The present invention provides a solution to this and other problems, and offers other advantages over the prior art.